ephemerals: allow root ephemeral outputs in stacks and test

[liamcervante]

Backport after 1.14.0 release.

We currently prevent root modules from exposing ephemeral values via outputs. This makes total sense as the ephemeral values shouldn't be saved or exposed anywhere.

However, in both stacks and terraform test, we sometimes pretend that a module is a root module when it isn't really. This PR disables the ephemeral check within Terraform Test and Terraform Stacks. For Stacks this is fine, as there are additional checks at the higher level to prevent stack outputs themselves being ephemeral. For Terraform Test we want people to actually test non-root modules, so we do need Core to support ephemeral outputs at the root module as the module being tested might not be intended to be consumed as a root module anyway.

Fixes #37805

Target Release

1.15.x

Rollback Plan

• If a change needs to be reverted, we will roll out an update to the code within 7 days.

Changes to Security Controls

Are there any changes to security controls (access controls, encryption, logging) in this pull request? If so, explain.

CHANGELOG entry

• This change is user-facing and I added a changelog entry.
• This change is not user-facing.

[jbardin]

My 👍 was assuming the failed test was a minor oversight, but the additional planning option seems like the only viable way to make this conditional at the moment.